<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <!--
    display:none; 让元素隐藏
    display:block;让元素显示
    -->
    <style>
        ol ol{display: none;}
        ol:hover ol{
            display: block;
        }
        div {
            background-color: #3c763d;
        }
    </style>
</head>
<body>
<div></div>
    <ol>
        <li>a</li>
        <li>
            a
            <ol>
                <li>b</li>\
                <li>b</li>\
                <li>b</li>\
            </ol>
        </li>
        <li>a</li>
        <li>a</li>
        <li>a</li>
    </ol>
</body>
</html>